The present invention relates to a system for measuring the amount of a specific substance mixed in a material under measurement from a measured value of conductivity of the material obtained with at least four electrodes. The present invention also relates to an electrode rod for measuring conductivity or resistivity by supplying an alternating current into a mixed material and measuring the resulting voltage using a group of one resistance and four electrodes.
The conductivity of water increases with increase in the amount of pollutants dissolved in the water. Therefore, the conductivity can be used as an indication of water pollution. In response to the growing interest in environmental protection, the measurement of conductivity is widely performed to detect the degree of water pollution. The conductivity of a material reflects the concentration of impurities contained in the material. Therefore, the measurement of conductivity can be expected to be applied not only to the field of detection of water pollution but also to the field of composition measurement.
The conductivity of soil increases with increase in the amount of water or salt water contained in the soil. The measurement of the conductivity of soil can be expected to be applied to various fields. Examples of applicable fields are detection of the amount of water for irrigation in irrigation cultivation; detection of a situation in which soil containing water weakens, which may lead to a landslide or other similar disaster; detection of the entry of salt water into cultivated land in a coastal region; and detection of the accumulation of salts in soil due to over-watering in a dry region.
There are two methods for measuring conductivity or resistivity, which is the reciprocal of conductivity, i.e. a method in which an electric current and a voltage are measured simultaneously by using only two electrodes, and a method using a group of four electrode to measure an electric current and a voltage independently of each other with respective pairs of electrodes. With the method using only two electrodes, measured data is influenced by a contact potential difference between the electrodes and the material under measurement. In contrast, the method using four electrodes to measure an electric current and a voltage independently of each other can eliminate the influence of contact potential difference.
Further, the measuring method using only two electrodes provides measured data reflecting only a value of conductivity or resistivity of the material under measurement present between the two electrodes, which contains the contact potential difference as an error. However, the measuring method using a group of four electrodes as one electrode system is advantageous in that when the material under measurement extends over a wide area as in the case of the ground, measured data can be made to reflect values of conductivity or resistivity over a wide area by appropriately positioning the electrodes. Further, if a large number of sensors are installed according to the extent of the ground, it is possible to measure the distribution of conductivity or resistivity over a wide area and to observe how the conductivity or resistivity changes with time.
According to the conventional method of measuring conductivity to detect the degree of water pollution, water as a material under measurement is interposed between two electrodes, and the conductivity is obtained as the reciprocal of resistivity between the electrodes. Accordingly, polarization due to electrochemical action occurs at the area of contact between each measuring electrode and water, which makes it impossible to obtain a correct value. Further, to measure a large number of samples, it is necessary to wash the electrodes with fresh water every time the samples are changed.
It is possible to obtain a value of conductivity free from the influence of polarization by using four electrodes in such a manner that two of the four electrodes are used to supply an electric current into a mixed material and the other two electrodes are used to measure an electric potential difference in the mixed material, and obtaining the ratio between the electric current and voltage measured with the respective electrodes (i.e. the measured electric current is divided by the measured voltage). With this method, however, it is not easy to perform division between two analog signals. Therefore, the conventional practice is to use a method wherein a dial is turned by a manual operation, and the dial value when the signal zero point is obtained is read off to obtain a value of conductivity, or a method wherein the measured voltage and electric current are read off as digital signals, and division is executed by a computer.
The resistivity of a material under measurement varies with the temperature of the material regardless of whether the number of electrodes used is two or four. Therefore, even when the composition of impurities in the material under measurement is constant, the conductivity, which is the reciprocal of the resistivity, varies undesirably.
Further, when a plurality of impurities are present in a material under measurement, the content of each impurity element contributes to the overall conductivity. Accordingly, it is impossible to exclusively measure the content of a specific ingredient of interest or changes in the ingredient content.
Thus, an accurate value cannot be obtained with the method using only two electrodes, although this method allows measurement to be carried out easily at site. On the other hand, the method using four electrodes requires much labor at site. Further, both the methods have the disadvantage that measured data is influenced by the temperature of the material under measurement and also by the contribution to the overall conductivity made by ingredients other than a specific ingredient of interest.
In resistivity measurement for the exploration of ground, a plurality of electrodes are horizontally placed in the ground surface to measure resistivity. Resistivity measurement is also performed for the following purposes: detection of the amount of water for irrigation in irrigation cultivation; detection of a situation in which soil containing water weakens, which may lead to a landslide or other similar disaster; detection of the entry of salt water into cultivated land in a coastal region; and detection of the accumulation of salts in soil due to over-watering in a dry region. For these purposes, an electrode system needs to be installed in the ground so as to extend in the direction of depth in order to measure the distribution and time variation of water content or salinity concentration in the direction of depth of the ground.
Accordingly, a large number of electrodes are buried in the ground in a state where they are mounted on a rod-shaped member to perform measurement at a large number of points in the direction of depth. In this case, there is a problem of how to deal with a large number of cables connected to the electrodes. That is, if the cables are routed along the outer wall of the electrode rod, the contact between the cylindrical electrodes and the surrounding soil is degraded. At the same time, the presence of a large number of cables in the soil undesirably influences the distribution and time variation of water content or salinity concentration in the material under measurement. Further, in outdoor measurement, the conductivity or resistivity of the material under measurement changes to a considerable extent with diurnal variations in the underground temperature and hence does not accurately follow the water content or the salinity concentration.
An object of the present invention is to simplify the operation to be performed at site and to allow temperature variations of a material under measurement to be reflected in the measurement of a current signal, thereby making it possible to perform conductivity measurement free from the influence of temperature variations.
Another object of the present invention is to improve the accuracy of measurement of the amount of a specific substance mixed in a material under measurement.
Still another object of the present invention is to make it possible to readily measure the distribution and time variation of conductivity or resistivity in the direction of depth of soil, a lake, a marsh, etc.
To attain the above-described objects, the present invention provides a system for measuring the amount of a specific substance mixed in a material under measurement. The system includes at least four electrodes placed in the material under measurement in a desired manner. A resistance for measuring an electric current is placed in the material under measurement together with the electrodes. A power supply supplies an alternating current between two of the at least four electrodes through the resistance. A current measuring unit measures an electric current flowing through the resistance. A voltage measuring unit measures an electric potential difference between electrodes of the at least four electrodes other than the electrodes supplied with the alternating current from the power supply. A computing unit obtains a conductivity on the basis of the electric current measured with the current measuring unit, the electric potential difference measured with the voltage measuring unit, and a coefficient determined by the manner in which the at least four electrodes are placed, and subjects the conductivity to conversion based on calibration data to obtain the amount of the specific substance of interest mixed in the material under measurement. The at least four electrodes and the resistance may be supported together as one unit on a support and placed in the material under measurement in a desired manner. The support is at least one plate- or rod-shaped member.
In addition, the present invention provides an electrode rod for measuring conductivity or resistivity by supplying an alternating current into a mixed material and measuring the resulting voltage using a group of one resistance and four electrodes. The electrode rod includes a distal end member formed from a cylindrical member, one end of which is formed into a conical shape. The cylindrical member is integrally formed at the other end thereof with an inner cylindrical portion having an outer diameter smaller than the outer diameter of the cylindrical member by an amount corresponding to the wall thickness of the cylindrical member. The electrode rod further includes a plurality of annular electrodes each having an inner diameter corresponding to the outer diameter of the inner cylindrical portion and a plurality of spacers each having an inner diameter corresponding to the outer diameter of the inner cylindrical portion. A cable lead-in opening is provided in the wall surface of the inner cylindrical portion. The plurality of annular electrodes are fitted onto the outer periphery of the inner cylindrical portion one after another at spacings determined by the spacers, and cables are led to the annular electrodes through the cable lead-in opening. The electrode rod may further include a coupling cylindrical member having the same outer diameter as the outer diameter of the inner cylindrical portion. The coupling cylindrical member is integrally formed at one end thereof with a coupling portion having an inner diameter corresponding to the outer diameter. The inner cylindrical portion may be coupled with the coupling cylindrical member to form a multi-stage structure. The plurality of spacers may include a spacer having a mounting portion for mounting a resistance for detecting an electric current on the wall surface thereof.
In addition, the present invention provides an electrode rod for measuring conductivity or resistivity by supplying an alternating current into a mixed material and measuring the resulting voltage using a group of one resistance and four electrodes. The electrode rod includes a cylindrical member with a conical distal end. Four annular electrodes are fixed on the outer periphery of the cylindrical member at predetermined spacings. A resistance for detecting an electric current is fixed between the annular electrodes. Preferably, the space between second and third annular electrodes of the four annular electrodes is enlarged, and the resistance is fixed between the second and third annular electrodes.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The invention accordingly comprises the features of construction, combinations of elements, and arrangement of parts which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims.